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Oxford AQA IGCSE Chemistry
A Simple Model of the Atom
Contents
The Elements
The Structure of the Atom
The Mass of Atoms
Isotopes
Energy Levels
Relative Atomic Mass
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The Elements
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Atoms & Elements
In chemistry, elements are substances that are made up of only one type of atom
The chemical elements are listed in the periodic table
A simple periodic table
The Periodic Table lists all (over 100) chemical elements
Atoms of each element are represented by a chemical symbol
For example, an atom of oxygen is represented by O
WORKED EXAMPLE
Complete the table to:
Identify the chemical symbol for an element
Identify an element from its chemical symbol
Element
Symbol
Element
C
Sulfur
Chlorine
Symbol
Na
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N
Aluminium
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Answers:
Element
Symbol
Element
Symbol
Carbon
C
Sulfur
S
Chlorine
Cl
Sodium
Na
Nitrogen
N
Aluminium
Al
Elements can be grouped into two categories:
Atoms - made from single particles
Molecules - made from more than one particle chemically joined together
Examples of elements
Elements contain only one type of atom and exist as atoms or molecules
Elements cannot be broken down into simpler substances by chemical means
But, they can be reacted together to make more complex substances
Elements are, therefore, considered to be the basic building blocks of all substances
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The Structure of the Atom
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The Atomic Model
How the atomic model has changed over time
In 1803, the atomic model proposed by John Dalton was a simple sphere of matter
At the time, the theory was correct but new experimental evidence, from scientific advances, led to the
atomic model developing over time
This is a fundamental feature of science: new experimental evidence leads to scientific models
being changed or replaced
The evolution of atomic models
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The atomic model has developed from a simple sphere to the current model
Atomic Structure
Atoms are tiny particles of matter, which are the building blocks of all matter
Each atom is made of subatomic particles called:
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1. Protons
2. Neutrons
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3. Electrons
The structure of a carbon atom
A carbon atom has a nucleus, containing protons and neutrons, surrounded by shells of electrons
Most of an atom is empty space
The centre of an atom is called the nucleus
The nucleus consists of protons and neutrons
The nucleus is positively charged
The electrons surround the nucleus in shells
Electrons have almost no mass, so most of the mass of an atom is located in the nucleus
EXAM TIP
The atom is the smallest part of an element that exists and still has the properties of the element
- the subatomic particles do not.
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Relative Charges of the Proton, Electron & Neutron
Protons, neutrons and electrons are so small that it is not practical to measure their charges
using conventional units, such as coulombs
Instead, their charges are compared to each other
This is why they are called relative electrical charges
The relative electrical charges of the subatomic particles are:
Table of relative electrical charges
Sub-atomic particle
Relative electrical charge
Proton
+1
Neutron
0 (neutral)
Electron
-1
Atoms have no overall charge
This is because they have the same number of positive protons as negative electrons
The negative charge of one electron exactly cancels out the positive charge of one proton
Ions are formed when an atom loses or gains electrons to achieve a full outer shell
If an atom loses one negative electron, it forms a positively charged 1+ ion
If an atom gains one negative electron, it forms a negatively charged 1- ion
WORKED EXAMPLE
Explain why a magnesium ion has a 2+ charge.
Answer:
A magnesium atom has:
12 positive protons
AND
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12 negative electrons
Magnesium loses 2 electrons to form a magnesium ion
This means it now has:
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12 positive protons
AND
10 negative electrons
Therefore, the overall charge is 2+
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The Mass of Atoms
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Relative Masses
Protons, neutrons and electrons are so small that it is not practical to measure their mass
using conventional units, such as grams
Instead, their masses are compared to each other
This is why they are called relative masses
Protons and neutrons have a very similar mass
So, they are both assigned a relative mass of 1
Electrons are roughly 2000 times smaller than a proton and neutron
So, the mass of an electron is described as very small or negligible
The relative masses of the subatomic particles are:
Table of relative masses
Sub-atomic particle
Relative mass
Proton
1
Neutron
1
Electron
very small
Atomic Number & Mass Number
Atomic Number
The atomic number (or proton number) is the number of protons in the nucleus of an atom
The symbol for this number is Z
The atomic / proton number is unique to each element, so no two elements have the same number of
protons
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Mass Number
The mass number is the total number of protons and neutrons in the nucleus of an atom
The symbol for this number is A
Representing Atoms
Every element is shown on the periodic table
Each element has its own symbol, mass number and atomic number and is represented as shown
Atomic Number & Mass Number diagram
Diagram showing the notation used on the periodic table
EXAM TIP
Both the atomic number and the mass number are given on the periodic table, but it can be easy to
confuse them.
Think MASS = MASSIVE, as the mass number is always the big number, the small number is therefore
the atomic number.
WORKED EXAMPLE
An element of sodium is shown on the periodic table as:
23 Na
11
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For an atom of sodium, state the following:
1. The number of protons
2. The number of protons and neutrons
Answer:
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An atom of sodium contains:
1. 11 protons
2. 23 protons and neutrons
Using the information from a chemical symbol, it is possible to calculate the number of protons,
neutrons and electrons
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Isotopes
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Isotopes
Isotopes are atoms of the same element that contain a different number of neutrons
Isotopes also contain the same number of protons and electrons
The symbol for an isotope is the chemical symbol (or word) followed by a dash and then the mass
number
For example, carbon-14 or C-14 has the chemical symbol 14 C
6
This means that it is the isotope of carbon with 6 protons
But, the 14 shows that it has 8 neutrons (14 - 6 = 8)
Isotopes display the same chemical properties
This is because they have the same number of electrons in their outer shells, and this is what
determines their chemistry
Table to show the structures of isotopes of hydrogen
Isotope
Atomic Structure
Symbol
Hydrogen 1
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Hydrogen 2
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Hydrogen 3
EXAM TIP
For atoms to be isotopes of each other, they must both be from the same element.
For example, carbon-13 and carbon-14 are isotopes whereas carbon-13 and hydrogen-2 are not.
Calculating PEN Numbers
Protons
The number of protons, p, is given by the atomic number
Electrons
Since atoms are neutral, the number of electrons, e, is the same as the number of protons
The number of electrons in an element is also given by the atomic number
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Ions have a different number of electrons to the number of protons, depending on their charge
A positively charged ion has lost electrons and therefore has fewer electrons than protons
A negatively charged ion has gained electrons and therefore has more electrons than protons
Neutrons
The mass number is the number of protons plus neutrons
The number of neutrons , n, can be calculated by:
Number of neutrons = mass number - atomic number
For example, the chemical symbol for an atom of beryllium is:
The chemical symbol for beryllium
Beryllium has an atomic number of 4
So, it has 4 protons
Beryllium is an atom, it has not gained or lost any electrons
So, it has 4 electrons
The mass number of beryllium is 9
So it has 9 - 4 = 5 neutrons
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So, the PEN numbers for beryllium are:
p=4
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e=4
n = (9 - 4 =) 5
WORKED EXAMPLE
Determine the number of protons, neutrons and electrons in:
1. An atom of carbon.
2. An Na+ ion.
Answers:
1. An atom of carbon contains:
6 protons
This is because the atomic number of carbon is 6
6 electrons
This is because an atom has no overall charge, so the number of protons = the number of
electrons
6 neutrons
The mass number of carbon is 12
Neutrons = mass number - atomic number
So, 12 - 6 = 6
2. An Na+ ion contains:
11 protons
This is because the atomic number of sodium is 11
10 electrons
This is because an Na atom would have 11 electrons but it has lost one electron to become
Na+
So, 11 - 1 = 10
12 neutrons
The mass number of sodium is 23
Neutrons = mass number - atomic number
So, 23 - 11 = 12
With the right information, the number of neutrons equation can be rearranged to determine the mass
number or atomic number, e.g.:
Mass number = Number of neutrons + atomic number
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Atomic number = Mass number - number of neutrons
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WORKED EXAMPLE
Calculate the number of sub-atomic particles in an unknown atom of element X with mass number
63 and 34 neutrons.
Protons: ..........
Electrons: ..........
Answer:
Protons: 29
Protons = mass number - neutrons
Protons = 63 - 34 = 29
Electrons: 29
The question is about an atom, so the number of electrons = the number of protons
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Energy Levels
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Energy Levels
Electrons orbit the nucleus in energy levels or shells
Each shell has a different amount of energy associated with it
The further away from the nucleus, the more energy a shell has
Electrons fill the shell closest to the nucleus
When a shell becomes full of electrons, additional electrons have to be added to the next shell
The first shell can hold 2 electrons
The second shell can hold 8 electrons
For this course, a simplified model is used where the third shell can hold 8 electrons
For the first 20 elements, once the third shell has 8 electrons, the fourth shell begins to fill
The outermost shell of an atom is called the valence shell
Atoms are more stable if they completely fill their outermost shell with electrons
Filling electron shells
A simplified model showing the electron shells
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The arrangement of electrons in shells can also be explained using numbers
Instead of drawing electron shell diagrams, the number of electrons in each electron shell can be
written down, separated by commas
This notation is called the electronic configuration (or electronic structure)
E.g. Carbon has 6 electrons, 2 in the first shell and 4 in the second shell
Its electronic configuration is 2,4
Electronic configurations can also be written for ions
E.g. A sodium atom has 11 electrons, a sodium ion has lost one electron, and therefore has 10
electrons; 2 in the first shell and 8 in the 2nd shell
Its electronic configuration is 2,8
The Electronic Configuration of the First Twenty Elements
Element
Atomic Number
Electronic Configuration
hydrogen
1
1
helium
2
2
lithium
3
2,1
berylium
4
2,2
boron
5
2,3
carbon
6
2,4
nitrogen
7
2,5
oxygen
8
2,6
fluorine
9
2,7
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neon
10
2,8
sodium
11
2,8,1
magnesium
12
2,8,2
aluminium
13
2,8,3
silicon
14
2,8,4
phosphorus
15
2,8,5
sulfur
16
2,8,6
chlorine
17
2,8,7
argon
18
2,8,8
potassium
19
2,8,8,1
calcium
20
2,8,8,2
Note: although the third shell can hold up to 18 electrons, the filling of the shells follows a more
complicated pattern after potassium and calcium
For these two elements, the third shell holds 8 and the remaining electrons (for reasons of stability)
occupy the fourth shell first before filling the third shell.
WORKED EXAMPLE
Draw and write the electronic structure of magnesium.
Answer:
Magnesium has 12 electrons in total.
A maximum of two can fit in the first shell and eight in the second shell.
The remaining two will occupy the third shell.
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The written form of this electronic structure is 2,8,2
EXAM TIP
It is a good idea to draw the electrons in their shells in pairs.
You will still score the marks if they aren't, as long as you have the correct number in each shell, but
this makes it easier for the examiner to count.
How does the electronic structure of an element relate to its
location in the Periodic Table?
There is a clear relationship between the electronic configuration and how the Periodic Table is
designed
The number of notations in the electronic configuration tells us the number of occupied shells
This tells us what period an element is in
The last notation shows the number of outer electrons the atom has
This tells us the group an element is in
Elements in the same group have the same number of outer shell electrons
Diagram showing the relationship between the electronic
configurations
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The electronic configuration for chlorine
Period: The red numbers at the bottom show the number of notations
The number of notations is 3
Therefore chlorine has 3 occupied shells
Group: The last notation, in this case 7
This means that chlorine has 7 electrons in its outer shell
Chlorine is therefore in Group 7
The Periodic Table showing the location of chlorine
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Chlorine is in Group 7, Period 3
EXAM TIP
The group number will be labelled on the Periodic Table you are given in your exam, but the period
number isn't so it is a good idea to write this on yourself at the beginning.
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Relative Atomic Mass
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Relative Atomic Mass
Relative atomic mass has the symbol Ar
This is the average mass of the atoms of an element measured relative to carbon-12
For example:
Carbon has an Ar of 12 and hydrogen has an Ar of 1 on the periodic table
So, 1 atom of carbon has the same mass as 12 atoms of hydrogen
Comparing hydrogen and carbon
Relative atomic mass compares atoms of other elements with carbon-12
EXAM TIP
Defining relative atomic mass is a common exam question.
The best answers work through the term backwards and include the required extra detail:
1. Mass - the average mass
2. Atomic - of an atom
3. Relative - compared to carbon-12
The relative atomic mass of any element is shown on the periodic table
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It is the mass number
The mass number that is shown on the periodic table is an average value of all the isotopes of an
element
This is calculated from the mass number and relative abundances of all the isotopes of a
particular element
The isotopes of carbon are carbon-12, carbon-13 and carbon-14
But, the percentage of carbon-12 is so high that the average value of all isotopes shown on the
periodic table is 12
Using isotopes for the Ar of carbon
The percentage of the carbon isotopes
The isotopes of chlorine are chlorine-35 and chlorine-37
The percentage of chlorine-35 is roughly three quarters of all chlorine atoms
So, the average value of all chlorine atoms shown on the periodic table is 35.5
Using isotopes for the Ar of chlorine
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The percentage of chlorine isotopes
EXAM TIP
You will not be asked to use isotopes and their abundances to calculate values for relative atomic
mass.
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